Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and as composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called “nano-scaled graphene plates (NGPs).” NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. Jang and Huang have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. Another process developed by B. Z. Jang, et al. [“Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004)] involves (1) providing a graphite powder containing fine graphite particles (particulates, short fiber segments, carbon whisker, graphitic nano-fibers, or combinations thereof) preferably with at least one dimension smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are either partially or fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled to obtain NGPs. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important industrial applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
In this and other prior art methods for making separated graphene platelets, the process begins with intercalating lamellar graphite flake particles with an expandable intercalation agent (intercalant), followed by expanding the intercalant to exfoliate the flake particles. Conventional intercalation methods and recent attempts to produce exfoliated products or separated platelets are given in the following representative references:                1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969).        2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).        3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).        4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).        5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).        6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).        7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).        8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).        9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).        
However, these previously invented methods had a serious drawback. Typically, exfoliation of the acid-intercalated graphite occurred at a temperature in the range of 800° C. to 1,050° C. At such a high temperature, graphite could undergo severe oxidation, resulting in the formation of graphite oxide, which has much lower electrical and thermal conductivities compared with un-oxidized graphite. In our recent studies, we have surprisingly observed that the differences in electrical conductivity between oxidized and non-oxidized graphite could be as high as several orders of magnitude. It may be noted that the approach proposed by Mack, et al. [e.g., Ref. 9, U.S. Pat. No. 6,872,330 and J. J. Mack, et al., “Graphite Nanoplatelet Reinforcement of Electrospun Polyacrylonitrile Nano-fibers,” Adv. Materials, 7 (2005) Jan. 6, pp. 77-80] is also a low temperature process. However, it involves intercalating graphite with potassium melt, which must be carefully conducted in vacuum or extremely dry glove box environment since pure alkali metals like potassium and sodium are extremely sensitive to moisture and pose an explosion danger. This process is not amenable to mass production of nano-scaled platelets.
It is therefore an object of the present invention to provide a method of exfoliating a laminar (layered) compound or element, such as graphite, graphite oxide (partially oxidized graphite), and graphite fluoride without involving a high exfoliation temperature.
It is another object of the present invention to provide a convenient method of exfoliating a laminar material to produce nano-scaled platelets (platelets with a thickness smaller than 100 nm and mostly smaller than 10 nm).
Another object of the present invention is to provide an effective and safe method of mass-producing nano-scaled platelets.
It is still another object of the present invention to provide a method of producing nano-scaled platelets that can be readily dispersed in a liquid to form a nanocomposite structure.